The invention is directed to a process for changing the speed of a vehicle with driving wheels and with a continuously adjustable speed/torque characteristic in the drive train, corresponding to a change in the position of an accelerator pedal carried out by the driver of the vehicle. Every position of the accelerator pedal is associated with a predetermined drive output P.sub.b (.alpha.)=f.sub.1 (.alpha.).times.P.sub.max and/or a predetermined driving torque M.sub.v (.alpha.)=f.sub.1 (.alpha.).times.M.sub.max as a predetermined value, wherein the predetermined value is used as a reference value for an electronic drive control of the vehicle. The invention is also directed to a vehicle for implementing this process.
Although electric motor drives have been common in rail-borne vehicles for decades, such drives have so far been used in road vehicles only in a few special cases. Particularly in the field of passenger automobile construction, this has generally involved experimental vehicles. In general, road vehicles continue to be outfitted with an internal combustion engine which acts on the driving wheels via a mechanical drive train. In order to utilize a range of the speed/torque characteristic of the internal combustion engine favorable for the driving conditions in question at different driving speeds, a manual or automatic transmission with a hydraulic torque converter is normally provided in the drive train. In this way, the operator or driver who initiates an acceleration process at a given speed of the vehicle by means of an appropriate change in the position of the gas pedal (accelerator) is afforded the possibility, by selecting the gear, to use different ranges of the speed/torque characteristic and accordingly different output stages for the acceleration process.
In electric motor drives, particularly in drives with DC motors having a permanently excited stator and rotor and a corresponding rotor or stator provided with electromagnets controlled via the output electronics, it is possible to adjust practically any optional speed/torque combinations within wide limits, which can be referred to in this sense as an infinitely variable or continuously adjustable speed/torque characteristic or speed/output characteristic. There are also vehicles with mechanical drive trains (CVT transmission) which offer similar freedom with respect to speed ratio or gear ratio variations.
In view of these special properties there is no need in such vehicles, even those having high top speeds (e.g., 200 km/h), for a manually or automatically actuated transmission in the drive train. But, in so doing, the operator of the vehicle is deprived of the possibility of substantially influencing the time required for carrying out a desired change in speed, i.e. the magnitude of the acceleration quantity, through a selection of a "driving gear" by actuating the gearshift lever or gear selector. The only possibility remaining for the driver to transmit a "driving selection" to the vehicle is that of actuating the accelerator pedal. This means that he has no particular way of conveying to the vehicle his desire for a determined target speed and his desire for the magnitude of acceleration to achieve this target speed, which desires must be viewed as mutually distinct.
Consequently, in known vehicles with electric motor drive the actual position of the accelerator pedal is interpreted as a determining quantity for the driving torque desired by the driver or for the desired drive output and is controlled directly corresponding to the characteristic of the drive. The selection of a determined driving torque M at a given speed n is synonymous with the selection of a determined drive output P in view of the following equation: EQU P=M.times.n.
As a result of an increase in the driving torque (or drive output) in effect in stationary driving operation (i.e. at constant speed), the vehicle is accelerated until the increased driving torque is again in equilibrium ("stationary operation") with the opposite moment produced by the actual driving resistance forces (particularly the air resistance and rolling resistance).
In the case of a conventionally driven vehicle outfitted with an internal combustion engine, the new driving torque desired by the driver is never achieved in a sudden manner, but rather is only reached gradually corresponding to the characteristic of the internal combustion engine as the speed increases, barring the case of a gear change in which a sudden change in the speed/torque combination is effected in a compulsory manner.
In a drive design with continuously adjustable speed/torque characteristic, particularly as it relates to electric motors as known, for example, from EP 0 094 978 A1, DE 34 14 314 A1, DE 38 16 651 A1 or WO 89/08346, the drive control of the vehicle must set the value to which the drive parameters represented by speed and torque or output are to be adjusted during the acceleration process, i.e. until the desired new driving state has been achieved. The transition from the (stationary) driving situation at the start of the acceleration process to the desired driving situation could always be effected, for example, with the maximum possible torque or maximum output. This would entail a constant "sporty" driving mode which many drivers would find uncomfortable. However, lower values could also be set for implementing such acceleration processes, although this would have the disadvantage that the vehicle would behave too sluggishly in situations where particularly high acceleration values are desirable (e.g. passing). In principle, this disadvantage could be overcome in that the driver informs the drive control of the vehicle of his desire for sporty driving or for a more comfortable driving style via a selector switch. However, the fact that the actuation of such a selector switch could be required under dangerous circumstances must be viewed as a severe disadvantage. Moreover, a drive design with continuously adjustable speed/torque characteristic could also meet with some resistance on the part of the driver if this would mean that the vehicle would behave in a fundamentally different manner than that to which he is accustomed from conventionally driven vehicles. This is also true for the case of "negative" acceleration, i.e. for applying a braking torque when the driver backs off the position of the accelerator pedal.
Further, a vehicle with a drive slippage regulating system (ASR) which has a stepped transmission in the drive train and in which an electronic control unit provides a special conversion of the respective position of the accelerator pedal actuated by the driver of the vehicle into an engine torque to improve driving stability is known from DE 37 27 690 A1. Outside of the regulating operation of the ASR, the engine torque, per se, is determined in a conventional manner in direct correspondence to a given characteristic by the respective position of the accelerator pedal. However, this correspondence is limited to the lower adjustment range of the accelerator pedal. For easier management of critical driving situations, e.g. due to a sudden change in the friction coefficient of the roadway, it is suggested that a second characteristic which assigns a value for the reference slippage of the driving wheels to the accelerator pedal position in question is provided for the upper adjustment range of the accelerator pedal. In so doing, the engine torque is given only indirectly, that is, as a function of the driving situation in question (friction coefficient, extent of lateral forces, etc.). There is no reference made in this patent to the set of problems involved in carrying out changes in the speed of vehicles with continuously adjustable speed/torque characteristic in the drive train by actuating an accelerator pedal.